There is known a power supply device (a single-phase three-wire inverter) including two single-phase inverters 100X and 200X that operate in series synchronization (see FIG. 4). The two inverters 100X and 200X are electrically insulated from each other except that outputs thereof are connected in series with each other. The inverters 100X and 200X have bridge circuits 11 and 21, respectively. The bridge circuit 11 converts a direct-current voltage “Vdc1” supplied to a primary side thereof into an alternating-current voltage having a desired amplitude and outputs the alternating-current voltage by FETs “Q11” to “Q14” performing respective switching operations in accordance with a PWM signal, and the bridge circuit 21 converts a direct-current voltage “Vdc2” supplied to a primary side thereof into an alternating-current voltage having a desired amplitude and outputs the alternating-current voltage by FETs “Q21” to “Q24” performing respective switching operations in accordance with a PWM signal. The power supply device outputs an alternating-current voltage that is twice as high as alternating-current voltages “Vac1” and “Vac2” output from the inverters 100X and 200X as an output voltage “Vout” across output terminals “OUT1” and “OUT2”.
The inverters 100X and 200X have peak limiter circuits 16X and 26X, respectively, and the peak limiter circuits 16X and 26X make the bridge circuits 11 and 21 stop operating, respectively, when an output current “Iout” reaches a positive limiter value and a negative limiter value. The limiter values of the two inverters 100X and 200X are independently determined. However, if the positive limiter values of the two inverters are equal to each other, and the negative limiter values of the two inverters are equal to each other, the bridge circuits 11 and 21 of the two inverters 100X and 200X stop operating when the output current “Iout” reaches any limiter value, and thereby can limit the maximum value of the output current “Iout” to the positive limiter value and the minimum value of the output current “Iout” to the negative limiter value. Therefore, damage to the FETs “Q11” to “Q14” and “Q21” to “Q24” of the bridge circuits 11 and 21 can be prevented in a situation where an excessive output current “Iout” can flow.
The inverters 100X and 200X have protecting circuits 18X and 28X, respectively, each of which forcedly stops both the inverters 100X and 200X to protect the power supply device when a short-circuit state in which the output current “Iout” reaches a limiter value has lasted for a predetermined time (several seconds, for example) or when the direct-current voltage “Vdc1” or “Vdc2” on a primary side becomes equal to or higher than an overvoltage detection value.
As a load on the power supply device, a load that requires high current in activation (a short-circuit start load), such as an incandescent lamp, can be connected to the power supply device. At the instant when current supply to such a load is started, the output terminals “OUT1” and “OUT2” of the power supply device are short-circuited, so that the output current “Iout” reaches the limiter value. However, once the load is activated, the impedance increases within the predetermined time described above, and therefore the output current “Iout” becomes lower than the limiter value. Therefore, if the positive limiter values of the two inverters 100X and 200X are equal to each other, and the negative limiter values of the two inverters 100X and 200X are equal to each other, the power supply device can keep supplying current to the load without stopping operating.
Another known device similar to the power supply device described above is described in Japanese Patent Laid-Open No. 2003-047296.